Adjusting Fuel Pressure for Better Running Engine, 1.7 Engine Performance Options

[R Shaff]

I've read some helpful things here while helping with my brother's '72 914. I think I've learned something new and thought I'd share it with you guys.

This car has the original engine and FI system, though the ignition system has been updated and is electronic. The car wasn't running well, and the local shop my brother uses said he should do a carb conversion, costing over $4,000. We looked online and found some good write-ups that recommended keeping the FI system. I've worked on many cars and restored one, so I thought I'd try to get the car running well enough with the original system.

The compression and ignition checked out fine. We adjusted the valves (of course). We tested and then replaced 3 injectors, and the CHT sensor. I put the fuel pressure at 28 psi, per spec. Static tests of the manifold pressure sensor came out fine. A cigar smoke test showed no intake leaks. We cleaned and adjusted the TPS, and then finally changed the TPS contact plate and adjusted the arm (when the car just wouldn't run right). All the other components checked out fine, using tests I found online.

The result? The engine fired right up, with a good idle. But when driving it lacked power, and the higher the rpm the less it wanted to pull. (Not right for a Porsche.) It felt like the timing wasn't advancing, or that the engine just wasn't getting enough air or gas in the right balance.

Other than buying a replacement MPS or computer, I was running out of ideas to try. One of good things about carbs is that they are adjustable. One of the challenging things about these old open-loop FI systems is that they are not really designed to be adjusted much, or at all. Repair is usually a process of testing and replacing components. But this car sure felt like it needed some kind of adjustment.

One of the odd things about this car, in my experience, is that the fuel pressure regulator IS adjustable. So that's what we tried, just using seat-of-the-pants performance as the indicator. We dialed the pressure down a turn and things got worse. Then we turned the pressure up, one turn at a time, and voila! As we did the car ran better and better. Now it pulls in the upper ranges like it should, much like it used to.

Now we're going to buy a 4th injector of the same type and adjust the pressure again as needed. (Now that we know we can adjust the whole system.) Perhaps that will make it run better still.

I haven't read that anywhere(about adjusting fuel pressure to get the engine to run better) so I thought it might help someone else who is struggling with these old Bosch systems.

Happy motoring!

Richard

[Minerva's 914]

Thanks Richard for the writeup. When Fat Performance built my 2.0L motor with a slight (80cc) increase in displacement, they suggested that I might need to increase the fuel pressure to get the most out of the motor but I couldn't find any reference to anyone doing this, so I was a bit unsure if I would benefit from do it and no longer having a FPG to read the pressure unnerved at shooting in the dark.

So, let me ask a question. Would I be correct in guessing that looking down on the adjustment screw turning clockwise (screwing in) would reduce pressure and turning counterclockwise (screwing out) would increase pressure?

Thanks again mate, and (IMG:style_emoticons/default/welcome.png)

Brady

[R Shaff]

Thanks Richard for the writeup. When Fat Performance built my 2.0L motor with a slight (80cc) increase in displacement, they suggested that I might need to increase the fuel pressure to get the most out of the motor but I couldn't find any reference to anyone doing this, so I was a bit unsure if I would benefit from do it and no longer having a FPG to read the pressure unnerved at shooting in the dark.

So, let me ask a question. Would I be correct in guessing that looking down on the adjustment screw turning clockwise (screwing in) would reduce pressure and turning counterclockwise (screwing out) would increase pressure?

Thanks again mate, and (IMG:style_emoticons/default/welcome.png)

Brady

Brady,

I found the opposite to be the case. Once you have loosened the locking nut buy a few threads (13mm nut, I think), then you turn the bolt in, clockwise, to increase pressure, and out to decrease pressure.

I had a pressure gauge on it when I first made the adjustment. It wasn't real sensitive to small turns. When I made adjustments this time, I did it in whole turn (360 degree) increments (corresponding to a thread on the bolt).

And of course, one needs to re-tighten the locking nut when done.

(Ive read that one can make small mixture adjustments with the knob on the ECU, which I have not tried yet. But those adjustments are supposed to only affect the mixture at idle. Not sure about that, but I'll try it too.)

I hope that helps.

[Minerva's 914]

Thanks Richard for the writeup. When Fat Performance built my 2.0L motor with a slight (80cc) increase in displacement, they suggested that I might need to increase the fuel pressure to get the most out of the motor but I couldn't find any reference to anyone doing this, so I was a bit unsure if I would benefit from do it and no longer having a FPG to read the pressure unnerved at shooting in the dark.

So, let me ask a question. Would I be correct in guessing that looking down on the adjustment screw turning clockwise (screwing in) would reduce pressure and turning counterclockwise (screwing out) would increase pressure?

Thanks again mate, and (IMG:style_emoticons/default/welcome.png)

Brady

Brady,

I found the opposite to be the case. Once you have loosened the locking nut buy a few threads (13mm nut, I think), then you turn the bolt in, clockwise, to increase pressure, and out to decrease pressure.

I had a pressure gauge on it when I first made the adjustment. It wasn't real sensitive to small turns. When I made adjustments this time, I did it in whole turn (360 degree) increments (corresponding to a thread on the bolt).

And of course, one needs to re-tighten the locking nut when done.

(Ive read that one can make small mixture adjustments with the knob on the ECU, which I have not tried yet. But those adjustments are supposed to only affect the mixture at idle. Not sure about that, but I'll try it too.)

I hope that helps.

Sweet! Exactly what I needed to hear. Right now I'm only about 20 miles into the break-in so I'll not be running it to high revs until I do the 1,000 mile oil change and valve adjustment but may start playing with the FP a little bit here. I would think the real value of an increase in FP would be at higher RPMs as the demands on fuel delivery increases so the real tale of the tape will come later.

Man this 914 stuff is fun! Might have to get another one!